The basic principles and techniques for electromagnetic logging for earth formations are well known. For example, induction logging to determine the resistivity (or its inverse, conductivity) of earth formations adjacent a borehole has long been a standard and important technique in the search for and recovery of subterranean petroleum deposits. In brief, a transmitter transmits via an antenna an electromagnetic (EM) signal that passes through formation materials around the borehole and induces a signal in one or more receivers. The amplitude and/or phase of the receiver signals are influenced by the formation resistivity, enabling resistivity measurements to be made. The measured signal characteristics and/or formation properties calculated therefrom are recorded as a function of the tool's depth or position in the borehole, yielding a formation log that can be used by analysts.
In order to protect the EM antenna from the mechanical stresses of drilling, the antenna is typically encased by an antenna shield. The antenna shield typically takes the form of a metal enclosure with adjacent apertures, slots or openings to allow the EM signal to propagate through the shield.